Don't Kick Harder!
No, No, No. . . sorry that I gave an ambiguous suggestion . . .my comment was meant to explain that there are other reasons to freedive with long blade fins other than purely for efficiency.
When freediving, you don't have a BCD, and so you become very negatively buoyant at depth. It is frustrating, and even a little frightening, to be at 60 or 70 or 100 feet, start to head back up, and find you have to kick like a maniac to get yourself moving toward the surface. . .that feeling of kicking rapidly and going nowhere.
Long blade fins have a larger surface area relative to other fins, and they displace more water with each kick and you will begin to move yourself toward the surface with a couple of firm kicks. Thus, long blades are not necessarily more efficient than other fins, because those couple of big kicks with a long blade take more energy than a couple of kicks with other fins, but you start moving!!
Now, at dynamic apnea depths, negative buoyancy is not the issue, but speed is . . .and long blades can increase your speed more rapidly than other fins . . .by displacing more water with each kick. . .but you pay a great price for speed because of the tremendous increase in resistance as speed increases . . .if I recall correctly, in water, resistance increases as the cube of the velocity. So dynamic apnea requires finding that fine line between efficiency of movement, speed of swimming, streamlining, and oxygen utilization. It is more complex than just how fast or slow you kick.
-Hope that makes more sense.